Alkali-support interactions on rubidium base catalysts determined by XANES, EXAFS, CO2 adsorption, and IR spectroscopy

Rubidium was supported on a variety of carriers, including magnesia, titania, alumina, carbon, and silica, by decomposition of an impregnated acetate precursor at 773 K. Except for Rb/MgO, none of the oxide-based samples contained surface carbonates as detected by IR spectroscopy after pretreatment at 773 K. The XANES and EXAFS associated with the Rb Kedge indicated that the local structure around Rb was highly dependent on the support composition. For example, the Rb-O distance was significantly shorter on carbon and silica compared to more basic carriers. Results from CO2 stepwise TPD showed that Rb/MgO possessed strongly basic sites that were not present on pure MgO. The basic sites formed by Rb addition to the other supports were weaker than those on Rb/MgO. Decomposition of 2-propanol at 593 K was used to relate catalytic activity to surface basicity. As anticipated, Rb/MgO and MgO were highly active and selective for alcohol dehydrogenation. Addition of Rb to alumina and titania greatly decreased the activity of the support oxides for the acid-catalyzed dehydration of 2-propanol. Reactivity and characterization results of Rb/SiO2 were consistent with the formation of a highly disordered, weakly basic, surface silicate phase that exhibited little activity for alcohol decomposition. The overall rate of acetone formation from 2-propanol correlated with the ranking of support basicity evaluated from the Sanderson intermediate electronegativity principle. These results suggest that strongly basic alkali-containing catalysts should utilize basic carriers to minimize alkali-support interactions that lower base strength. (C) 1997 Academic Press.